Polyurethane foams are widely used as materials from which articles such as mattresses, seat cushions, and thermal insulators are fabricated. Such polymeric foam materials are ordinarily manufactured by a casting process in which a mixture of liquid polyurethane-foam-generating reactants are deposited in a mold. As used herein, the term "mold" includes both stationary molds for batch casting and translating or otherwise moveable molds for continuous casting. Evolution of a gas causes the reactants to foam. For some foam formulations, the reactants themselves react to evolve sufficient gas; in others, a blowing agent is mixed with the reactants to provide gas evolution. Continued gas evolution causes the foam to expand to fill the mold. The foam, initially a fluid froth (the term "froth" as used by applicant herein means a more viscous, partially-expanded, less dense, prefoamed fluid mixture obtained from the reaction of a liquid mixture of polyurethane foam generating reactants.), becomes increasingly viscous as the reactants polymerize, ultimately curing into a polyurethane foam casting shaped by the mold.
Slabs of polyurethane foam approximately rectangular in cross section are conventionally cast in a translating channel-shaped mold. Such molds typically include a belt conveyor forming the bottom of the mold and a pair of spaced-apart, opposing side walls, which can be fixed or translatable at the speed of the conveyor. The mold sides and bottom are generally lined with one or more sheets of flexible-web such as kraft paper or polyethylene film. The sheets of mold liner are ordinarily withdrawn from rolls and continuously translated along the mold channel at the same speed as the belt of the conveyor. Liquid foam-generating reactants are deposited on the mold bottom in a zig-zag pattern from a nozzle positioned above the mold which is reciprocated back and forth across the width of the mold. Typically, as the foam expands, the reactants flow together and merge into a uniform slab of foam.
If fresh reactant mixture is deposited on top of foam generated from previously deposited reactants, the resulting cured foam will have an uneven surface and nonuniform density, which is undesirable for most applications. By continuously translating the mold liner, the reactant mixture is continuously carried away from the pouring area below the pouring nozzle, which reduces the tendency for fresh reactant mixture to cover previously deposited mixture.
To reduce further the tendency of the liquid reactants to flow back under the pouring nozzle and to assist the "zig-zags" of reactant mixture to merge uniformly, it is customary to incline the surface under the nozzle from horizontal so that the bottom liner slopes downward in the direction of translation. However, the angle of inclination of the pouring board cannot be greater than about 4.5.degree. from horizontal for typical flexible polyether polyurethane-foam formulations without causing the reactant mixture to flow forward under previously deposited mixture, which leads to undesirable nonuniform foam. The maximum angle of inclination is different for different foam formulations, such as polyester polyurethane foams.
Problems arise if the mold bottom slopes downward along its entire length. Conventional continuous slab molds are quite long, typically in excess of 60 feet, to provide for the long curing time of the foam. Building a translatable mold of this length inclined from horizontal is significantly more expensive than building a translatable mold of the same length which is horizontal, since, for example, the building housing the inclined mold would be required to have higher than normal ceilings. Moreover, it is especially expensive to provide for changing the angle of inclination of the entire mold to compensate for differing viscosities among the various foam formulations. Thus some continuous slab molds have horizontal belt conveyors for most of the length of the mold bottom, but have relatively short inclined pouring boards located beneath the pouring nozzles. The expansion and rise of the foam generally takes place on the sloping pouring board.
A second reason for providing a pouring board which makes an angle with respect to the belt conveyor concerns the cross-sectional shape of the slab cast in the mold. As the foam expands and rises in the mold, it encounters the sides of the mold. If the mold-side liners are being translated exactly parallel to the mold bottom, the expanding foam experiences a shear force which resists its rise along the sides. This shear force results in a rounding of the top of the slab to form a crown or crest of convex shape, much like a loaf of bread. For most applications such rounded portions are unusable and must be discarded as scrap. Thus the more nearly rectangular the cross section of the slab, i.e., the flatter the top, the more economical is the casting process.
If, over the length the foam travels as it expands, the mold bottom liner and the two mold side liners are translated, not in parallel, but at an angle with respect to one another, the mold side liner can have a velocity component relative to the mold bottom in the direction of the expansion of the foam which can compensate for the shear force which resists the rise of the foam. Guiding the mold bottom liner an inclined pouring board which is located between the side walls of a slab mold and intersects the mold-bottom conveyor at an angle can provide such a compensating velocity component when the foam expansion is carried out over the length of the pouring board and mold-side liners are translated parallel to the mold-bottom conveyor. The angle of intersection which ordinarily leads to polyurethane foam slabs having the most nearly rectangular cross sections is about 10.degree. for typical foam formulations and production conditions. Unfortunately, if the pouring board is sloped 10.degree. from horizontal, freshly deposited reactant mixture tends to flow forward, as discussed above, leading to foam slabs of nonuniform density or otherwise imperfect.
Although it is possible to construct a continuous slab mold with a pouring board inclined from horizontal by an angle of 4.5.degree. and intersecting the belt conveyor at 10.degree., the belt conveyor in such a case must be inclined upward by an angle of 5.5.degree.. See, for example, my U.S. Pat. No. 3,325,823. As noted above, however, inclined translatable molds are more expensive than comparable horizontal molds.
U.S. Pat. No. 3,786,122 discloses a process for producing polyurethane foam slabs which employs a horizontal, channel-shaped mold having at its forward end an inclined "fall plate" which makes an angle of significantly greater than 4.5.degree. from horizontal. The problem of reactant mixture flowing down the inclined fall plate is obviated by prereacting the reactant mixture prior to introducing it onto the fall plate. The prereacting step is carried out in a trough which opens onto the upper edge of the fall plate. Liquid foam reactants are introduced into the bottom of the trough and the foam which is generated is allowed to expand upwards in the trough and spill over onto the fall plate. The foam continues to expand as it is carried down along the fall plate by a translating bottom sheet. Since the prefoamed reactant mixture existing the trough is more viscous than the initial liquid reactant mixture, the fall plate can be inclined at a greater angle from horizontal than a pouring board in a conventional polyurethane-foam slab mold.
An additional result of introducing prefoamed reactant mixture into the mold is that relatively high foam slabs can be produced as compared with conventional processes. The height to which foam rises can be thought of as being divided into two components, a first component is the result of the expansion of the foam below a horizontal plane passing through the point at which the reactants begin to foam and is determined by the decline and length of the pouring board, and a second component is the result of the rise of the foam above the horizontal plane.
Economies result from producing high slabs since, the thicker the foam slab, the less is the loss from discarding the rind which generally coats polyurethane foam castings. With a conventional slab mold, if the rate of introduction of reactant mixture is kept constant and the rate of translation of the mold liner is reduced, the height of the foam slab tends to increase since more foam-generating reactant is deposited per unit length. However, since the rate of gas evolution remains essentially constant, the rising of the foam takes place over a shorter linear distance, in addition to rising to a greater height, which gives the rising foam a steeper slope. If the rate of translation is slowed sufficiently, this slope becomes so steep that the expanding foam, particularly the youngest and most fluid portion, becomes unstable and tends to slip and shift, which results in cracks and other imperfections in the cured foam.
This problem of instability of rising foam is reduced in the process of the U.S. Pat. No. 3,786,122 by introducing into the translating mold prefoamed reactant mixture which is sufficiently viscous as to be able to sustain a relatively steep slope of the pouring board as it completes its expansion. Thus the first component which determines the height of the foam can be increased. In addition to permitting higher foam slabs to be cast by reducing the translation speed of the mold liner, this process permits the use of slab molds shorter than those of conventional processes, since the slab moves a shorter distance during the curing time.
In practice, however, the process of the U.S. Pat. No. 3,786,122 suffers from a number of drawbacks. The prefoamed reactant mixture introduced into the mold must be quite fluid, since the foaming mixture rising in the trough must, by gravity flow, spill over a weir structure and onto the fall plate of the mold. Thus prefoamed reactants which are too viscous to flow freely cannot be used. This limits the height of slabs which can be obtained by the process.
Additional problems attend the use of the open trough of the U.S. Pat. No. 3,786,122. For example, changing the width of the trough is difficult because foam deposits interfere with reestablishing fluid-tight seals. Moreover, the trough opening is subject to partial blockage by deposits of cured foam along the back and sides where the flow of prefoamed reactant mixture stagnates. Such deposits break free from time to time and are swept over the weir into the rising foam, thereby causing objectionable nonuniformities in the foam slab.
A further difficulty is encountered when air bubbles are introduced into the bottom of the trough with the liquid reactants. These air bubbles generally remain entrained in the foam, leading to voids and other defects in the cured material.